Three of the countries on that chart are islands. Australia, New Zealand, and UK. Islands have more control over their borders and trade. Its harder to smuggle in guns. Banning civilians from owning guns in the US will make citizens more vulnerable to crime and attack. Guns will be smuggled into the country via Mexican broder, just like drugs are. Drugs are banned yet I can obtain marijuana easier than alcohol. People in South American countries who can't own guns build massive fortresses around their houses to prevent burglaries. They live in fear. Fear of police, fear of criminals.
Homogeneous, what I ment is those countries have a high specific majority of similar people. Whether its similar in religion, race, or political views. This may be wrong but my theory is that US is such a diverse country that there is more likely disagreements which can produce violence. Right-wing vs left-wing, religions, ethnic stuff like Mexican gangs vs Cuban gangs vs Russian gangs, etc.